Two British-designed desalination plants have exploded as they neared 250 hours of operation and an order for ten plants in the United States now hangs in the balance. Suspecting sabotage, the Bishop assigns Unwin and Matthew to ensure that the last remaining plant, at Port Trennick, stays operational.

Professor Soames has developed a sonic rifle, capable of projecting destructive waves of ultrasonic vibration, accurate to within a centimetre over thirty feet. But three unscrupulous saboteurs intend to use Soames' rifle to bring down an experimental aircraft, holding the Professor's daughter hostage to ensure his complicity. When Father Unwin becomes suspicious and investigates, he too is held prisoner in Soames' house.

Father Unwin discovers that a gang of bogus vicars are responsible for a series of sabotage attacks on military installations and vehicles. After one ofthe gang is hospitalised after their latest venture, Unwin masquerades as his replacement, a demolition expert, to infiltrate the gang, but he is soon exposed as an imposter.